Generally, culturing cells have been performed using a culture medium. The culture medium is a culture solution containing a large amount of nutritive substances. When the time of using the culture medium is longer, the pH of the culture medium is decreased due to influences such as lactic acid produced by cell growth. When the pH of the culture medium is decreased, the pH is out of a range suitable for cell growth and the cell culture state may become inactive. Among conventional apparatuses for culturing cells, there is an apparatus that replaces a culture medium at regular intervals according to the kind of cell, in order to keep the pH range of the culture medium constant (for example, refer to PTL 1).
The apparatus disclosed in PTL 1 extrapolates the pH of the culture medium from changes in color of phenol red contained in the culture medium, and replaces the culture medium when the extrapolated pH is determined to be unsuitable for cell growth.
FIG. 6 is a diagram showing a relationship between absorbance and pH in the apparatus disclosed in PTL 1. The apparatus disclosed in PTL 1 measures the pH from the absorbance using the relationship shown in FIG. 6 and gives an instruction to replace the culture medium when the measured pH is less than a predetermined value.